Pressure compensated pyrotechnic time delay composition

ABSTRACT

THIS INVENTION RELATES TO A PYROTECHNIC TIME DELAY COMPOSITIONS WHICH COMPRISES TUNGSTEN, POTASSIUM PERCHLORATE, BARIUM CHROMATE, DIATOMACEOUS EARTH, AND A METAL FLUORIDE SELECTED FROM THE GROUP CONSISTING OF MAGNESIUM FLUORIDE, CERIC FLUORIDE, STRONTIUM FLUORIDE, AND CALCIUM FLUORIDE.

United States Patent Int. Cl. C06b 11/00 US. Cl. 149-40 Claims ABSTRACTOF THE DISCLOSURE This invention relates to a pyrotechnic time delaycomposition which comprises tungsten, potassium perchlorate, bariumchromate, diatomaceous earth, and a metal fluoride selected from thegroup consisting of magnesium fluoride, ceric fluoride, strontiumfluoride, and calcium fluoride.

BACKGROUND OF THE INVENTION Compressed columns of solid powder mixtureshave long been used for initiating some desired pyrotechnic result atthe end of a predetermined time delay period. The earliest form of suchpyrotechnic device consisted of a confined column of black powder whichcould be ignited at one end so that the flame and glow front couldadvance along the column to the other end at a substantially uniform andpredetermined rate.

So called delay mixtures have been developed consisting of powderedmanganese, silicon, boron, and zirconium nickel alloys in combinationwith solid oxidizers, such as barium chromate, lead chromate, andpotassium perchlorate. Many of these existing type delay mixtures aresubject to a number of serious limitations. Very often the range ofburning time per lineal inch is restricted to a narrow range, i.e. 10-13seconds. Many are subject to undesirable variations in burning rateresponsive to changes in pressure and temperature. Many are notignitable in a rarified atmosphere or, at least, do not burnconsistently under such conditions. In addition, very often theelectrical conductivity of the mixture, both before and after thepyrotechnic reaction, is low, and this is undesirable when the delaytrain is to be used in multistage rockets and similar modernapplications.

In an effort to overcome these disadvantages Olander, in Letters PatentNo. 3,028,229 formulated pyrotechnic time delay compositions which wereeasily ignitable, were consistently burning, were chemically stable,were supposedly relatively nneifected by temperature and pressure, had abroad range of burning times and were electrically conductive. Olanderachieved these results by combining either chromium, molybdenum ortungsten With potassium perchlorate and a second oxidizer, such asbarium chromate; with the quantity of potassium perchlorate being lessthan the stoichiometric equivalent of the metal powder present in themixture.

Olander used chromium and molybdenum mixtures to produce compositionswhich have a burning rate between about one and about ten seconds perinch. Tungsten was used to form compositions with a burning rate ofabout 40 seconds per inch. However, the tungsten compositions of Olanderdo not produce a burning rate of over 40 seconds per inch and for manypurposes a slower burning 3,701,697 Patented Oct. 31, 1972 rate would bedesirable. For example, when one is presented with space limitations butstill desires a considerable time delay, one can decrease the columnlength if one can also decrease the burning rate. Changing of theconcentration or the particle size of the metal has been used as astandard method for burning rate control of tungsten powders. However,with decreasing tungsten concentration or increasing particle size ofthe metal, the ignitability of the system is reduced. Therefore,attempts to obtain compositions burning slower than 40 seconds per inchhave been unsuccessful. Furthermore, even with the addition of siliceousmatter, such as diatomaceous earth, for a beneficial effect on thecontrol of compaction and upon the reproducibility of the burning rate,the pyrotechnic time delay composition of Olander has the disadvantagethat its burning rate is effected by increases in pressure aboveatmospheric.

SUMMARY OF THE INVENTION It is an object of this invention to producepyrotechnic time delay compositions which have all of the desiredproperties of previously known compositions, and, in,

addition, which produce a slower burning rate than said compositions.

These and other objectives are accomplished by incorporating up to about5 percent of a chemically inert material such as calcium fluoride,magnesium fluoride, ceric fluoride or strontium fluoride into tungstencontaining compositions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Investigations on thecombustion of tungsten delay powders have shown that oxidation oftungsten at the boundary between unreacted and reacted material is arate controlling factor in the burning process. From these results a newmethod to obtain slow burning compositions has been derived; i.e. by theformation of a molten layer on the surface of tungsten during burning.While this can be accomplished by raising the burning temperature abovethe melting point of tungsten trioxide,

it is preferably accomplished by incorporating a low melting materialinto the composition. The low melting material must be chemically inertand must have a melting point below the burning temperature of thepyrotechnic composition. Some of the suitable materials, which can beused are either calcium fluoride, magnesium fluoride, ceric fluoride,strontium fluoride, or mixtures thereof. It is preferred to use calciumfluoride.

The compositions of the instant invention contain potassium perchlorateas a primary oxidizer, tungsten, di-

atomaceous earth, barium chromate as a secondary. oxidizer and a lowmelting material. Up to 5 percent of the fluoride, based on the weightof the composition, can

be incorporated into any of the tungsten compositions of US. Pat. No.3,028,229. Generally, the amount of powdered or finely divided tungstencan vary from about 10-90 percent by weight of the composition, and theamount of tungsten present will determine the time of burning. Theamount of potassium perchlorate varies from about 4.5 to about 15percent, with the proviso that the quantity of potassium perchlorate isless than the stoichiometric equivalent of tungsten powder present inthe mixture. The amount of barium chromate, which acts as a reactionmodifier to slow the burning rate, will vary from about 10 to about 60percent. The amount of diatomaceous earth will vary from about to about12 percent by weight of the composition. An especially desirablecomposition will contain 3 percent of diatomaceous earth and 3 percentof calcium fluoride. If the entire amount of diatomaceous earth presentin the compositions of Olander is replaced by 5 percent of calciumfluoride, loading characteristics become worse and consolidation of thepowder in the delay test vehicle becomes difiicult. However, such acomposition, while not preferred, is still within the scope of thisinvention. Thus, it is generally preferred to have from about 3 to about12 percent of diatomaceous earth present. The following compositions areexemplary of the many combinations which may be made without departingfrom the principle of our invention.

The above composition was prepared according to Schedule A, set forthhereinafter, and dried at 200 F. for a minimum of 8 hours. The batchesused were limited in production size, i.e. 2000 to 12,000 grams. Thecompositions were preblended in 1 gallon capacity glass jars by rollingon a jar rolling mill for about 15 minutes. The amount of material in ajar never exceeded 3000 grams. Mulling was done in a Simpson mix-mullerfor 30 minutes, with scraping down every 10 minutes for the 12,000 grambatches and in at Lancaster mixer for minutes for the 2000 and 2500 grambatches. The composition was then forced through a 30 mesh USS sieve anddried for a minimum of 24 hours at 140 F. before loading and testfiring.

Schedule A Ingredients: Preparation Tungsten 325 mesh USS sieve. Bariumchromate 30 mesh USS sieve. Potassium perchlorate 100+200 mesh USSsieve. Diatomaceous earth mesh USS sieve. Calcium fluoride -100 mesh USSsieve.

The resulting composition, in a vented system, had a burning time of47.508 seconds/inch at 71 F.

EXAMPLE II The composition of Example I was altered, in that acomposition was formulated containing tungsten and 59% barium chromate(with the other ingredients remaining constant). The composition had aburning time at 71 F. of 43.1814 seconds/inch.

EXAMPLE III The composition of Example I was altered in that acomposition was formulated containing 32% tungsten and 52% bariumchromate, with the other ingredients remaining constant. The compositionhad a burning time of 22.655 seconds/inch at 71 F. While this burningrate is within the range of the burning rates of U.S. Pat. No.3,028,229, due to the amount of tungsten and chromate present, theburning rate is still slower than a corresponding composition withoutfluoride.

The effect of pressure on a system containing calcium fluoride asopposed to a system without calcium fluoride is shown as follows. Thefiring test performed on the composition of this example was repeated onan obturated system rather than upon a vented system. Whereas the ventedsystem produced a burning rate of 22.655 seconds/ inch, the obturatedsystem with its increased pressure, produced a burning rate of 18.274seconds/inch. Thus, the percent difference between the vented andobturated burn times is 19.34%. This is compared with a composi- EXAMPLEIV This example shows the effect of replacing 5 percent of diatomaceousearth with 5 percent of calcium fluoride on the burning rates of variouscompositions containing 10 percent potassium chlorate and a balance ofbarium chromate.

Burning rate with- (a) CaFz Thus, a much slower burning rate is achievedby replacing diatomaceous earth in the composition of U.S. Pat. No.3,028,229, with calcium fluoride.

The pyrotechnic time delay compositions of the instant invention can beused wherever time delay compositions have heretofore been used, such asin cartridges, fuzes, ordnance devices requiring time delay, etc.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. A pyrotechnic time delay composition comprising tungsten, potassiumperchlorate, barium chromate and a low melting compound selected fromthe group consisting of calcium fluoride, strontium fluoride, cericfluoride, magnesium fluoride, and mixtures thereof, wherein the amountof potassium perchlorate is less than the stoichiometric equivalent ofthe tungsten present in said composition.

2. The composition of claim 1 wherein the composition additionallycontains diatomaceous earth.

3. A composition according to claim 2, which contains, on a weightpercent basis; from about 10-90 percent of tungsten, from about 4.5 toabout 15 percent of potassium perchlorate; from about 10 to about 60percent barium chromate; from about 3 to about 12 percent ofdiatomaceous earth; and up to 5 percent of said low melting compound.

4. A composition according to claim 1 wherein said low melting compoundis selected from the group consisting of calcium fluoride, strontiumfluoride, magnesium fluoride and ceric fluoride.

5. A composition according to claim 1 wherein said low melting compoundis calicum fluoride.

6. A composition according to claim 3 wherein said low melting compoundis calcium fluoride.

7. A composition according to claim 6 wherein both the calcium fluorideand the diatomaceous earth are present in the amount of about 3 weightpercent.

8. A composition according to claim 7, containing:

Percent Tungsten 32 Barium chromate 52 Potassium perchlorate 10Diatomaceous earth 3 Calcium fluoride 3 6 9. A composition according toclaim 7, containing: References Cited Percent UNITED STATES PATENTSTungsten 24 2,457,860 1/1949 Bennett et a1 149 40 Bamm? chmmate 52,696,429 12/1954 Hart 149 40X Pqtassmm Pmhlorate 10 2,830,885 4/1958Kerr et a1 149 40 X D1at9maeuSFarth 3 3,028,229 4/1962 Olander 149 40Calcwm flumde 3 3,172,795 3/1965 Helliwell et a1. 149 4o 0. A '1' d t 1i s, t 1 composl Ion accor mg 0 c a m s t 10 BENJAMIN R. PADGE'IT,Primary Examiner BI'CEH Tungsten 25 S. J. LECHERT, JR., AssistantExaminer Barium chromate 60 Potassium perchlorate 10 X'R' Calciumfluoride 5

